Embodiments of the present invention relate to power line communication (PLC) and, more particularly, to an optimized narrowband orthogonal frequency division multiplex (OFDM) based physical (PHY) frame structure.
Powerline communications (PLC) include systems for communicating data over the same medium that is used to transmit electric power to residences, buildings, and other premises. Once deployed, PLC systems may enable a wide array of applications, including, for example, automatic meter reading and load control for utility-type applications, automotive uses such as charging electric cars, home automation for controlling appliances and lights, and computer networking for internet of things (IoT).
Various PLC standardizing efforts are currently being undertaken around the world, each with its own unique characteristics. Examples of competing PLC standards include the IEEE 1901.2, HomePlug AV, and ITU-T G.hn (e.g., G.9960 and G.9961) specifications. Generally speaking, PLC systems may be implemented differently depending upon local regulations and characteristics of local power grids. For example, the U.S. FCC implementation of IEEE 1901.2 uses OFDM subcarriers from 10 kHz to 490 kHz. CENELEC, the European standard, has various implementations using OFDM subcarriers from 3 kHz to 148.5 kHz. ARIB, the Japanese standard, uses OFDM subcarriers from 10 kHz to 450 kHz. Another standardization effort includes, for example, the Powerline-Related Intelligent Metering Evolution (PRIME) standard designed for OFDM-based (Orthogonal Frequency-Division Multiplexing) communications. The current or existing PRIME standard is the Draft Standard prepared by the PRIME Alliance Technical Working Group (PRIME R1.3E) and earlier versions thereof.
Current and next generation narrowband PLC standards are directed to multi-carrier based systems, such as orthogonal frequency division multiplexing (OFDM) in order to get higher network throughput. OFDM uses multiple orthogonal subcarriers to transmit data over frequency selective channels. A conventional OFDM structure for a data frame includes a preamble, followed by a physical layer (PHY) header, a media access control (MAC) header, followed by a data payload. However, PLC channels are highly challenging environments for digital communication because they suffer from periodic bursts of impulse noise, and the channel impulse response also varies over time.
A conventional synchronization preamble structure for a narrowband OFDM PLC standard, such as IEEE 1901.2 (G3), includes 8 SYNCP symbols followed by 1.5 SYNCM symbols. The synchronization symbols are typically transmitted at a higher (3 dB) rms voltage than the data payload, and there is no cyclic prefix between adjacent symbols. Each SYNCP symbol is a known preamble sequence of different subcarriers phase shifted by a multiple of π/8. Subcarriers of the SYNCM symbol are phase shifted by π with respect to SYNCP so that SYNCM=−SYNCP. For example, a SYNCP symbol may be a chirp-like sequence of a specific binary sequence of 1s and −1s or a constant amplitude, zero autocorrelation (CAZAC) sequence. The definition of the SYNCP symbol for the FCC band in IEEE P1901.2 is defined in section 6.6 for specific subcarriers or tones.
The preamble serves several purposes including: 1) indicating to other nodes in the PLC network that a transmission is in progress; 2) determining the frame boundary between the preamble and the PHY header, and between the PHY header and the data payload; 3) determining accurate channel estimates; and 4) for frequency offset compensation. SYNCM symbols help determine the frame boundary and indicate the end of the preamble sequence. The repetitive SYNCP symbols also assist in preamble detection as receiver nodes are looking for the repetitive sequence of symbols in the PLC channel to determine whether or not a frame is on the powerline. Multiple SYNCP symbols also help in obtaining more accurate channel estimates by averaging the channel estimates across multiple symbols to reduce noise. Improved channel estimates also help in improving the header decoding performance when the header is coherently modulated with respect to the SYNCP preamble.
While preceding approaches provide improvement and standardization in PLC operation, the present inventors recognize that still further improvements are possible. This is particularly true for high data rate PLC applications. Accordingly, the preferred embodiments described below are directed toward this as well as improving upon the prior art.